In the art of making silicon nitride it is conventional to add certain oxides to the raw material from which the fully dense silicon nitride body is constituted; such oxides act as pressing aids or sintering aids (see U.S. Pat. No. 4,143,107). The presence of these oxides has required higher temperatures and pressures to reach full densification during hot pressing or sintering than what is optimally desired for a more economical process.
Generally, compounds other than oxides have been introduced to silicon nitride only as a result of chemical reaction during hot pressing or sintering (see U.S. Pat. Nos. 4,102,698; 4,341,874; and 4,350,771).
Thus, the prior art has recognized that secondary phases form from the oxides and ingredients of the basic mixture during hot pressing or sintering to produce silicates, complex oxides, and oxynitrides. With continuing research herein, it has been discovered that certain of these oxynitrides are more desirable than other oxynitrides and more desirable than silicates or oxides in promoting lower temperatures and pressures needed for processing. This selectively is due in part to the fact that certain of these compounds have a higher degree of solubility for silicon nitride, thus tending to promote a reduction in temperature and pressure needed for full dissolution of the silicon nitride during the heat fusion step.
What is needed is the ability to introduce chemical modifications to the mixture prior to hot pressing or sintering, which modifications allow for: (a) a reduction in the time and temperature required to hot press or sinter the mixture to a fully densified object; (b) freedom to increase the amount of silicate forming oxide to a higher controlled amount to optimize physical characteristics in the final object; and (c) closer control or selection of the desired secondary phase chemistry in the final product without total reliance on the vagaries of the chemical reaction during hot pressing or sintering.